From Type II to Z-Scheme: A DFT Study of Enhanced Water Splitting in the SGa₂Se/TeMoS Heterojunction

Abstract

Harnessing solar energy for photocatalytic water splitting and hydrogen fuel production necessitates the development of advanced photocatalysts with broad solar spectrum absorption and efficient electron-hole separation. In this study, we systematically explore the potential of the SGa₂Se/TeMoS heterojunction as a water-splitting photocatalyst using first-principles calculations. Our results indicate that while the heterojunction exhibits type-II band alignment, its band edge positions are inadequate for initiating water redox reactions. To overcome this limitation, we successfully engineered a Z-scheme SGa₂Se/Zr/TeMoS heterojunction by incorporating a Zr layer to modulate the charge transfer mechanism between the SGa₂Se and TeMoS layers. The potential positions of the HER and OER in this Z-scheme heterojunction overcome the limitation of the bandgap on water decomposition, allowing the optimized heterojunction to exhibit suitable band edge positions for water splitting across a wide pH range (0 ≤ pH ≤ 11.3), from acidic to weakly basic conditions. Additionally, the heterojunction exhibits exceptional light absorption capabilities across the entire spectrum, particularly in the infrared and visible regions, which greatly enhances the utilization of solar energy and highlights its potential as an efficient broad-spectrum photocatalyst for water splitting.

1. Introduction

In recent years, sustainable energy conversion technologies have garnered significant attention as viable solutions to address pressing environmental concerns and energy crisis. Photocatalytic water splitting, in particular, has emerged as a promising approach for generating clean hydrogen fuel through solar energy utilization, offering a sustainable pathway to mitigate environmental pollution and reduce carbon emissions [1,2,3]. This process necessitates photocatalysts with suitable band edge positions to facilitate both oxidation and reduction reactions during water splitting [4,5]. Specifically, the valence band maximum (VBM) [6] must exhibit adequate potential to drive water oxidation for oxygen evolution, while the conduction band minimum (CBM) [7] requires sufficient reduction potential to enable proton reduction for hydrogen generation [8]. These fundamental requirements underscore the importance of developing adequate photocatalytic materials for water splitting applications.

Transition metal dichalcogenides (TMDs) represent a prominent class of photocatalytic materials characterized by their distinctive layered structure, where strong covalent bonds dominate within individual layers while van der Waals forces govern interlayer interactions [9,10]. The unique electronic structure of TMDs, which can be precisely modulated through strategic selection of transition metal elements and control of layer number, has positioned these materials at the forefront of photocatalysis research [9,11]. Furthermore, the construction of van der Waals heterostructures by stacking different two-dimensional (2D) material monolayers has emerged as an effective strategy for combining the unique properties of each material. This approach significantly enhances charge transfer and separation processes, thereby improving photocatalytic efficiency [12]. Guo et al. [13] demonstrated that van der Waals heterostructures comprising InX (X = S, Se) and transition metal dichalcogenides exhibit type-II band alignment, enabling effective spatial separation of photogenerated charge carriers. This electronic configuration enhances charge carrier mobility, suppresses recombination rates and consequently, improves photocatalytic efficiency. Through careful control of material composition and stacking order, these heterojunctions can achieve band edge positions for water-splitting reactions [14,15]. Yang et al. [16] employed first-principles calculations to investigate the electronic and optical properties of MoSSe and Ga₂SSe Janus homojunctions by varying the number of layers (n = 1, 2, 3). The results revealed significant layer-dependent variations in electronic structures, directly influencing band alignment configurations and, consequently, water-splitting capabilities. Furthermore, the optical properties and solar-to-hydrogen conversion efficiencies exhibited pronounced layer-dependent characteristics, demonstrating the tunability of photocatalytic performance through layer number control.

Heterojunctions are typically classified into four categories based on their energy band arrangements: type-I, type-II, type-III and Z-scheme configurations [17,18,19]. Among these, type-II and Z-scheme heterostructures have demonstrated particular efficacy in photocatalytic water decomposition. Type-II configurations facilitate the spatial separation of photogenerated electrons and holes across different materials, effectively reducing recombination rates and enhancing photocatalytic efficiency. However, their performance is inherently limited, as the migration of charge carriers to lower energy states inevitably compromises their redox capabilities [20]. Inspired by natural photosynthetic systems, researchers have developed Z-scheme heterostructures to address the limitations of type-II configurations [21]. While both systems share similar energy band arrangements, their charge transfer mechanisms differ fundamentally [22,23]. Z-scheme heterojunctions offer superior performance in photocatalytic water splitting by maintaining higher redox potentials and enhancing charge separation efficiency [24,25]. Recent experimental studies have demonstrated the exceptional performance of Z-scheme systems. Chen et al. [26] reported a g-C₃N₄/rGO/PDIP Z-scheme heterojunction that achieved remarkable hydrogen and oxygen evolution rates of 15.80 and 7.80 μmol h⁻¹, respectively, representing a 12.1-fold enhancement compared to pristine g-C₃N₄. This system also exhibited an 8.5-fold improvement in charge separation efficiency, yielding a quantum efficiency of 4.94% at 420 nm and a solar-to-hydrogen conversion efficiency of 0.30%. Li et al. [27] developed a redox-mediator-free Z-scheme system using a 2D Janus bilayer V_s-ZnIn₂S₄/WO₃ heterostructure, where the W-S bonds facilitated enhanced charge separation and transport dynamics. To further improve the performance of this system, they integrated NiS quantum dots, resulting in novel 2D/2D NiS/V_s-ZnIn₂S₄/WO₃ heterostructures with exceptional stability, achieving a visible-light hydrogen evolution rate of 11.09 mmol g⁻¹ h⁻¹ and an apparent quantum efficiency of 72% at 420 nm.

In the present study, we systematically investigated the photocatalytic water-splitting potential of a novel SGa₂Se/TeMoS heterojunction through comprehensive density functional theory (DFT) calculations. Although this heterostructure exhibits a type-II band alignment, our analysis revealed that its band edge positions are not well-suited for overall water splitting due to incompatible redox potentials. While type-II configurations promote charge separation, their inherent limitation lies in the diminished redox ability of the separated carriers. To overcome this drawback, we developed a strategic approach consisting in the insertion of an ultrathin intermediate layer to convert the initial type-II configuration into a Z-scheme heterojunction, thereby establishing an optimal charge transfer pathway for efficient photocatalytic water decomposition under illumination. This transformation significantly enhances the charge transfer pathway while preserving the strong redox potentials characteristic of Z-scheme systems. In our case, the insertion of a zirconium (Zr) atomic layer between the SGa₂Se and TeMoS monolayers enables this transition, giving rise to a SGa₂Se/Zr/TeMoS Z-scheme heterostructure with suitable band edge alignment for photocatalytic water splitting. Furthermore, the modified heterojunction demonstrated a significant enhancement in light absorption across both the infrared and visible regions, underscoring its potential as an efficient photocatalyst for water splitting. The choice of zirconium [28] is motivated by its intermediate work function, which facilitates directional charge transfer between the two semiconducting layers. Additionally, its hexagonal close-packed (hcp) structure ensures good structural compatibility with the layered nature of the adjacent dichalcogenides, minimizing interfacial strain and promoting stable stacking. These findings provide valuable insights into the design of high-performance photocatalysts for hydrogen production.

2. Computational Methods

In this study, all computational calculations were performed using density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP) [29,30]. The exchange–correlation interactions were treated using the Perdew-Burke-Ernzerhof (PBE) functional within the framework of the generalized gradient approximation (GGA) [31,32]. Ion–electron interactions were described through the projector-augmented wave (PAW) method [33]. To accurately account for van der Waals interactions, we incorporated the DFT-D3 correction method developed by Grimme [34,35]. A plane-wave cutoff energy of 450 eV was set to fix the basis set size. Convergence criteria were defined as 10⁻⁵ eV for energy and 0.05 eV Å⁻¹ for forces. Brillouin zone integration was performed using a 4 × 4 × 1 k-point mesh, and a vacuum layer of 20 Å was implemented along the z-direction to eliminate spurious interactions arising from periodic boundary conditions. To ascertain the quality of the results with respect to the k-point grid and cutoff energy, the variation of the total energy as a function of these criteria is presented in Figure 1a,b for the SGa₂Se/TeMoS heterojunction. As can be seen, beyond the 4 × 4 × 1 k-point grid and 450 eV cutoff energy, which are the parameters selected for our study, the energy varies by less than approximately 5 meV. For the precise determination of electronic structures and optical properties, we employed the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional [36]. Furthermore, atomic charge distributions were quantitatively analyzed using the Bader charge analysis method [37].

3. Results and Discussion

3.1. Geometric and Electronic Structures of the Heterojunction

The TeMoS monolayer represents a distinctive two-dimensional material characterized by a metal atomic layer covalently bonded to two different chalcogen layers (tellurium and sulfur), as schematically illustrated in Figure 2a. Similarly, the SGa₂Se monolayer exhibits a unique structure comprising two metal atomic layers linked through metallic bonds (Figure 2b), with each metal atom forming covalent bonds with chalcogen atoms (sulfur or selenium). Both materials crystallize in a hexagonal structure with P6₃/mmc space group symmetry, exhibiting equivalent lattice constants a and b. Structural optimizations yielded lattice constants of 3.30 Å for TeMoS and 3.71 Å for SGa₂Se, which are consistent with previously reported values [38,39]. Electronic band structure calculations performed using the HSE06 hybrid functional (Figure 3a,b) revealed indirect bandgaps of 1.89 eV and 2.95 eV for TeMoS and SGa₂Se, respectively. To construct the heterojunction, we expanded the TeMoS monolayer into a 2 × 2 × 1 supercell and aligned it with the SGa₂Se supercell using a $\sqrt{3}\times \sqrt{3}\times 1$ adjustment to minimize lattice mismatch. The resulting heterojunction is depicted in Figure 2c. This configuration achieved a lattice mismatch of less than 5%, which is well within the acceptable range for stable heterojunction formation.

The projected band structure (Figure 3c) shows that the valence band maximum (VBM) primarily originates from the TeMoS layer, while the conduction band minimum (CBM) is dominated by the SGa₂Se layer. This electronic configuration confirms the formation of a type-II band alignment with an indirect bandgap of 1.38 eV, making the SGa₂Se/TeMoS heterojunction particularly suitable for photocatalytic water-splitting applications.

To gain deeper insight into the electronic properties of the SGa₂Se/TeMoS heterojunction, we analyzed its electrostatic potential distribution along the z-axis. As shown in Figure 4a, the heterojunction exhibits an electrostatic potential difference (ΔΦ) of 0.41 eV. According to the theoretical framework proposed by Li et al. [40], the bandgap requirement for polar materials can be redefined as E_g > 1.23 − ΔΦ eV, where ΔΦ represents the electrostatic potential difference (with ΔΦ > 0). Furthermore, the standard redox potentials for the H⁺/H₂ and O₂/H₂O couples in aqueous solutions can be expressed as follows [41]:

$$E_{{\mathrm{H}}^{+}/{\mathrm{H}}_2}=-4.44+0.059\times \mathrm{pH}$$

(1)

$$E_{{\mathrm{O}}_2/{\mathrm{H}}_2\mathrm{O}}=-5.67+0.059\times \mathrm{pH}$$

(2)

At pH = 0, the reduction potential of water is −4.44 eV, while the oxidation potential is −5.67 eV. After applying the necessary corrections, these values become −4.85 eV and −5.67 eV, respectively. As illustrated in Figure 4b, the band edge positions of the heterojunction demonstrate that the oxygen evolution reaction (OER) occurs at the VBM of the TeMoS layer, whereas the hydrogen evolution reaction (HER) takes place at the CBM of the SGa₂Se layer. While the band edge alignment satisfies the oxidation potential requirement for water splitting at pH = 0, it fails to meet the reduction potential criterion. Consequently, the heterojunction is unsuitable for photocatalytic water splitting under these conditions. Figure 4c depicts the charge density difference in the heterojunction, where the yellow and blue regions correspond to electron accumulation and depletion, respectively. According to the Bader charge analysis, a charge transfer of 0.019|e| occurs from the TeMoS layer to the SGa₂Se layer. This result supports the electron transfer mechanism associated with the type-II band arrangement.

3.2. Mechanism of Transformation from Type-II to Z-Scheme Heterojunction

To achieve effective modulation of the charge transfer mechanism in the SGa₂Se/TeMoS heterojunction, we introduced a Zr layer between the SGa₂Se and TeMoS monolayers (Figure 5a), resulting in the formation of a novel SGa₂Se/Zr/TeMoS heterojunction (Figure 5b). The thermodynamic stability of this heterojunction was evaluated using the following equation [42]:

$$E_{int}=E_{{\mathrm{S}\mathrm{G}\mathrm{a}}_2\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{r}/\mathrm{T}\mathrm{e}\mathrm{M}\mathrm{o}\mathrm{S}}-E_{{\mathrm{S}\mathrm{G}\mathrm{a}}_2\mathrm{S}\mathrm{e}}-E_{\mathrm{TeMoS}}-E_{\mathrm{Zr}}$$

(3)

where $E_{{\mathrm{S}\mathrm{G}\mathrm{a}}_2\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{r}/\mathrm{T}\mathrm{e}\mathrm{M}\mathrm{o}\mathrm{S}}$ represents the total energy of the heterojunction system, while $E_{{\mathrm{S}\mathrm{G}\mathrm{a}}_2\mathrm{S}\mathrm{e}}$, $E_{\mathrm{TeMoS}}$, and $E_{\mathrm{Zr}}$ correspond to the total energies of the constituent monolayers. The calculated interfacial interaction energy (E_int) of the SGa₂Se/Zr/TeMoS heterojunction is −2.27 eV, indicating strong stabilizing interactions between the constituent layers [43].

As illustrated in Figure 6a–c, the work function of the TeMoS monolayer is 4.48 eV, which is lower than that of the Zr layer (4.71 eV) and the SGa₂Se monolayer (5.90 eV). This work function difference facilitates the formation of a unique Z-scheme charge transfer pathway mediated by the Zr interlayer, as schematically shown in Figure 6d. To elucidate the underlying mechanism, we systematically analyzed the SGa₂Se/Zr and TeMoS/Zr interfaces separately. At the TeMoS/Zr interface, the higher Fermi energy level of TeMoS relative to the Zr layer results in the formation of a Schottky junction [44]. In this configuration, electron transfer from the TeMoS layer to the Zr one requires overcoming an energy barrier, while hole migration occurs freely. Conversely, at the SGa₂Se/Zr interface, the high work function (low Fermi energy level) of the SGa₂Se layer leads to the formation of an Ohmic junction [45], enabling efficient electron transfer from the SGa₂Se layer to the Zr one. This configuration ultimately establishes a Z-scheme heterojunction with enhanced redox potentials, promoting highly efficient photocatalytic water splitting.

The electronic band structure and the band edge alignment of the Z-scheme SGa₂Se/Zr/TeMoS heterojunction are illustrated in Figure 7a and Figure 7b, respectively. The inclusion of a Zr layer between the SGa₂Se and TeMoS layers induces a semiconductor-to-metal transition, closing the band gap of this heterojunction. Nevertheless, in the Z-scheme heterojunction, the OER occurs at the VBM of the SGa₂Se layer, while the HER takes place at the CBM of the TeMoS layer. The VBM of SGa₂Se and the CBM of TeMoS straddle the redox potentials of water, thereby enabling photocatalytic water splitting despite the absence of a band gap. Figure 7c shows the charge density difference in this heterojunction. It can be observed that electrons accumulate primarily in the S layer of the TeMoS layer. A quantitative analysis of interlayer electron transfer using Bader charge analysis reveals that electrons are transferred from the SGa₂Se layer to the Zr layer, and then from the Zr layer to the TeMoS layer, with a net charge transfer of 0.804 |e|. This charge redistribution results in the band edge positions satisfying the requirements for the redox reactions of the water across a wide pH range, from acidic conditions (0 ≤ pH ≤ 7) to weakly alkaline environments (7 ≤ pH ≤ 11.3), demonstrating the heterojunction’s potential for efficient photocatalytic water splitting under various pH conditions.

3.3. Optical Properties

The light absorption capacity, a crucial factor determining photocatalytic water splitting efficiency, was systematically investigated. As shown in Figure 8, we compared the absorption spectra of the SGa₂Se/Zr/TeMoS heterojunction with those of the individual SGa₂Se and TeMoS monolayers. The absorption coefficient (α) was calculated using the following dielectric function formula [46]:

$$\alpha \left(\omega \right)=\sqrt{2}\omega {\left[\sqrt{{ϵ}_1^2\left(\omega \right)+{ϵ}_2^2\left(\omega \right)}-{ϵ}_1\left(\omega \right)\right]}^{1/2}$$

(4)

where the real (ε₁) and imaginary (ε₂) components of the dielectric function are intrinsically related through the Kramers–Kronig transformation, with ε₂ being derivable from ε₁. Optical absorption analysis reveals distinct spectral characteristics: the SGa₂Se monolayer exhibits limited light absorption, while the TeMoS monolayer demonstrates strong absorption in the visible region. Remarkably, the SGa₂Se/Zr/TeMoS heterojunction shows substantially enhanced absorption coefficients throughout the entire spectral range compared to its constituent monolayers. Particularly noteworthy is the heterojunction’s exceptional absorption capabilities spanning from the infrared to the visible light regions, demonstrating its significant potential as an efficient photocatalyst for water splitting applications.

4. Conclusions

In conclusion, this study demonstrates the successful design of a Z-scheme SGa₂Se/Zr/TeMoS heterojunction as a highly efficient photocatalyst for solar-driven water splitting. Through first-principles calculations, we investigated the electronic properties of the SGa₂Se/TeMoS heterojunction and identified its limitations in band edge alignment for water redox reactions. By strategically incorporating a Zr layer, we designed a heterojunction with a Z-scheme charge transfer mechanism. This mechanism not only optimizes the band edge positions, thereby allowing the heterojunction to straddle the water redox potentials and facilitating both the HER and OER but also broadens the operational pH range for water-splitting reactions, extending it from 0 to 11.3. Remarkably, the modified heterojunction significantly improves solar energy utilization efficiency throughout the solar spectral range, particularly in the infrared and visible regions. These findings provide a strong theoretical support for the design of advanced heterojunction-based photocatalysts aimed at sustainable and efficient solar–hydrogen energy conversion.